Modern large-scale electronics, such as telecommunication or main-frame computing systems, are usually comprised of processing, access and/or memory functions. These processing, access and/or memory functions are often physically isolated modules, located on shelves of a cabinet, rack or like storage means. The modules on each shelf are interconnected by way of transmission lines on a large printed circuit board (PCB) referred to as a backplane (BP) located at the rear of the cabinet. If there are many modules, and hence many shelves, the storage means can be large and therefore the backplane can be a meter or more in length.
The transmission line interconnect is often called a bus, the bus being one or more conductors carrying data or control signals. The bus can be a point-to-multipoint or a multipoint-to-point interconnect. Indeed, the bus is usually connected to one or more signal sources, often called drivers, and one or more receivers. All connections can occur at the same or at multiple points along the bus.
Furthermore, the interconnection may be a differential bus to connect many differential electronic components. Differential electronic components are quite popular, and they use differential signals for inputs, outputs, or both. A differential signal is composed of two constituent signals of opposite polarity. Hence, the two constituent signals of a differential signal vary in phase by 180 degrees. Thus, differential electronic components have two terminals for each differential input port or differential output port. Differential electronic components respond to a difference in signals at the terminals for each port. If the same signal is incident at both terminals of a port, then this situation is defined as common mode. In common mode the difference between the signals at the two terminals of the port is zero, hence the differential electronic component does not respond. This is common mode rejection for differential electronic components.
The need for greater interconnect signal throughput has resulted in the need for the backplane to carry interconnect lines running at data rates upwards of 1 Giga-bits-per-second (Gbps). However, at such data rates, the interconnect must be considered as a transmission line with the propagating signal possessing wave properties. The wave properties will affect the signal quality along the transmission line. Impedance discontinuities and transmission line structure, for example, can cause signal reflections and local energy storage, respectively, which degrade the signal quality.
To address reflections arising from impedance discontinuities along the interconnecting transmission line, the traditional approach is to eliminate the impedance discontinuities causing the reflections. The impedance discontinuities are eliminated by ensuring that all element interfaces in the system are matched. By matched, it is meant that for each interface in the system, the impedance on one side of a given interface is the complex conjugate of the impedance on the other side of the same interface.
Often, the characteristic impedance of the bus, denoted by Z.sub.0, is defined as the impedance of the system. In such cases, to accomplish the matching, all of the elements connected to the bus are matched to the characteristic impedance of the bus to eliminate reflections. Furthermore, the end of the bus itself is terminated by its characteristic impedance to eliminate reflections from the end of the bus itself.
In light of the above issues, conventional hardwired interconnects are not feasible because they cannot be perfectly terminated in the line characteristic impedance, in order to damp reflections produced at the end of the interconnect.
In addition to the reflection issues, reliability requirements of a point-to-multipoint bus favour isolation between each receiver and the bus. If this is the case, a catastrophic failure at one receiver does not impact the entire bus. Accordingly, one possible backplane may carry many point-to-point interconnects to achieve the required isolation, thereby increasing the number of backplane signal layers. A better solution to alleviate the reliability and reflection problems simultaneously, is point-to-multipoint transmission line interconnects achieved using AC coupling, also referred to as non-contact coupling, as disclosed by De Veer et al. in U.S. Pat. No. 3,619,504. This type of interconnect disclosed by De Veer et al. is called a non-contact bus.
De Veer et al. disclose a high-speed data transmission network employing a point-to-multipoint transmission line interconnect. Directional coupling elements are spaced along the transmission line to separate the signals detected by each receiver using non-contact coupling. The directional coupling elements themselves are short sections of transmission line placed in very close parallel proximity to the main signal transmission line traversing the backplane.
Typically, when the data transmission network disclosed by De Veer et al. is employed, the coupler lines are fed through a backplane via connector to a receiver in one of the modules mentioned above. At data rates in excess of 1 Gbps, the connector and attendant mounting vias represent impedance mismatch and can lead to signal quality problems due to reflections at the receiver. In addition, the required connector pins are at a premium. These problems are especially prevalent in point-to-multipoint buses, since there will be many impedance mismatches along the bus.
Sometimes, as in a connector, the interfaces cannot be perfectly matched. Hence, to reduce the performance impact of these impedance mismatches, ideally both the receiver and coupler end of the bus is terminated in the coupler's characteristic impedance, and that impedance is substantially equal to the characteristic impedance of the bus. This characteristic impedance is often a pure resistance, hence the desired termination is a resistor with resistance substantially equal to the characteristic impedance. Thus, reflections due to impedance mismatches are absorbed by elements in the system and do not substantially degrade the signal quality.
Osaka et al. (H. Osaka, M. Umemura and A. Yamagiwa, "1 GT/s Back Plane Bus (XTL:Crosstalk Transfer Logic) using Crosstalk Mechanism", Hot Interconnects Symposium V 1997, Stanford University, CA, Aug. 21-23, 1997) have disclosed a non-contact bus with acceptable signal quality by terminating the couplers at both ends with resistors substantially equal to the characteristic impedance of the couplers.
Often, however, this type of coupler termination is undesirable since the resistors either have to be mounted directly on the backplane or in a separate module. Backplane mounted components are undesirable for space and reliability reasons, and module mounted components require the coupler transmission line to pass through a connector to find the termination. It is therefore desirable to terminate the couplers without using a resistor substantially equal to the characteristic impedance of the coupler, and still maintain acceptable signal quality at the receivers in a point-to-multipoint bus.